Yi San, later King Jeongjo (Lee Jun Ho) was the son of Crown Prince Sado and grandson of King Yeongjo .
So, if you want to know what happened between Crown Prince Sado (Yoo Ah-in) and King Yeongjo (Song Kang-ho) then this is the movie for you.

1) Both dramas have the same director
2) If you want to watch more about King Taejong (Yi Bang Won) who was the third king of the Joseon Dynasty in Korea and the father of King Sejong the Great. He helped his father King Taejo establish the Joseon Dynasty.